Method for the production of partially fluorinated fluoropolymers

ABSTRACT

The present invention relates to a process for the production of partially fluorinated fluoropolymers, in accordance with which fluoromonomers are free-radically polymerized in the presence of polyfluoropropanes or polyfluorobutanes of the formulae CF3-CH2-CF2H, CF3-CHF-CF2-H, CF3-CH2-CF3, CHF2-CF2-CH2F, CF3-CH2-CF2-CH3 AND CF3-CHF-CF2-CH3 and optionally a chain-transfer agent.

FIELD OF THE INVENTION

The present invention relates to a process for the production ofpartially fluorinated fluoropolymers, in accordance with whichfluoromonomers are free-radically polymerized in the presence ofpolyfluoropropanes or polyfluorobutanes of the formulae CF₃—CH₂—CF₂H,CF₃—CHF—CF₂H, CF₃—CH₂—CF₃, CHF₂—CF₂—CH₂F, CF₃—CH₂—CF₂—CH₃ andCF₃—CHF—CF₂—CH₃ and optionally a chain-transfer agent.

BACKGROUND OF THE INVENTION

Fluoropolymers are produced on an industrial scale substantially usingknown aqueous emulsion or suspension polymerization processes, asdescribed, for example, in Modern Fluoropolymers, John Wiley & SonsLtd., Chichester, 1997, pp. 77 and 609. Water-soluble auxiliaries, suchas fluorinated emulsifiers, dispersants, initiators etc. are required inthese processes which, once the resultant polymer dispersion has beenworked up, may be found in part in the product and in part in the wastewater, where they cause problems with regard to product properties orthe environmental compatibility of the process.

Non-aqueous processes have hitherto been based upon chlorofluorocarbons,such as 1,2-dichlorotetrafluoroethane or 1,1,2-trichlorofluoroethane.However, these compounds have an elevated ozone degrading potential, forwhich reason industrial use thereof is already prohibited in manyindustrial nations.

U.S. Pat. No. 4,243,770 mentions the possibility of using other inertcompounds containing fluorine, such asperfluoro(1,2-dimethylcyclobutane), perfluorocyclohexane,perfluoro(tributylamine) and compounds of the type H(CF₂)_(n)H andCF₃O(C₂F₄O)_(n)CF₂CF₃, apart from the stated chlorofluorocarbons, assolvents for the polymerization of fluoromonomers.

U.S. Pat. No. 5,182,342 describes the use of fluorinated hydrocarbonswhich satisfy certain criteria with regard to the F/H ratio and theposition of the hydrogen atoms and may optionally contain ether oxygenatoms for this purpose, such as for example1,1,2,2-tetrafluorocyclobutane,1-trifluoromethyl-1,2,2-trifluorocyclobutane,CF₃(CF₂)_(n)CFH(CF₂)_(m)CF₃, CF₃(CF₂)_(n)CFHCFH(CF₂)_(m)CF₃,CF₃(CF₂)_(n)CH₂(CF₂)_(m)CF₃, CF₃(CF₂)_(n)CFHCH₂(CF₂)_(m)CF₃,C₄F₉CH₂CH₂C₄F₉. However, compounds which, with the exception of thepermitted structural unit —CF₂OCH₃, contain a hydrogen atom on theterminal (primary) C atom are explicitly excluded.

Fluorinated hydrocarbons having terminal hydrogen atoms, for exampleCF₃(CF₂)_(n)H, CF₃(CF₂)_(n)CH₂CH₃, as well as those of the typeCF₃CF(CF₃)CFHCFHCF₃, are described in U.S. Pat. No. 5,494,984 inaddition to those mentioned above as polymerization media forfluoromonomers.

It has not hitherto been possible to use fluorinated hydrocarbons havingfewer than 4 C atoms in known processes for the production offluoropolymers. The compounds hitherto used have also been very highlyfluorinated compounds, in which partially fluorinated fluoropolymers,which also comprise —CH₂ sequences in addition to fluoro-substitutedcarbon atoms, are very sparingly soluble. Moreover, on grounds of priceand availability, none of these compounds is suitable as apolymerization medium. The production thereof generally proceeds byoligomerization/telomerisation of fluoromonomers (tetrafluoroethylene,hexafluoropropene, hexafluoropropene oxide) and subsequent hydrogenationor hydrogen fluoride addition.

However, increasing the H/F ratio as a pre-requisite for improvingpolymer solubility generally results in an increase in the transferpotential, which is undesirable for a polymerization solvent, as thislatter property is usually also associated with a terminating chaintransfer.

SUMMARY OF THE INVENTION

The object of the present invention was accordingly to provide a simpleproduction process for partially fluorinated fluoropolymers whichoperates without using ozone-damaging compounds.

It has now been found that partially fluorinated fluoropolymers may beproduced in a very simple manner by using certain polyfluoropropanes orpolyfluorobutanes.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a process for the production of partiallyfluorinated fluoropolymers, in accordance with which fluoromonomers arefree-radically polymerization in the presence of fluorinatedhydrocarbons of the formulae CF₃—CH₂—CF₂H, CF₃—CHF—CF₂H, CF₃—CH₂—CF₃,CHF₂—CF₂—CH₂F, CF₃CH₂—CF₂—CH₃ and/or CF₃—CHF—CF₂—CH₃, preferably1,1,1,3,3-pentafluoropropane, and optionally a chain-transfer agent.

Partially fluorinated fluoropolymers for the purposes of the inventionhave a main chain formed of carbon atoms, which chain also comprises—CH₂ sequences as well as fluoro-substituted sequences. These may beeither homo- or copolymers of vinylidene fluoride with other fluorinatedand also non-fluorinated monomers or copolymers of fluorinated monomers,containing no hydrogen, with non-fluorinated monomers.

Fluoromonomers which may be used for the purposes of the invention arefluorinated, optionally substituted ethylenes, which, apart fromfluorine, may contain hydrogen and/or chlorine, such as for examplevinylidene fluoride, tetrafluoroethylene and chlorotrifluoroethylene,fluorinated 1-alkenes having 2-8 carbon atoms, such as for examplehexafluoropropene, 3,3,3-trifluoropropene, chloropentafluoropropene,hexafluoroisobutene and/or perfluorinated vinyl ethers of the formulaCF₂═CF—O—X, where X=C₁-C₃ perfluoroalkyl or —(CF₂—CFY—O)_(n)—R_(F),wherein n=1-4, Y=F or CF₃ and R_(F)=C₁-C₃ perfluoroalkyl.

Olefins containing no fluorine, such as ethylene, propene, isobutene,alkyl vinyl ethers or vinyl esters, such as for example vinyl acetate,which are copolymerizable with fluoromonomers may furthermoreadditionally be used as comonomers.

Examples of such copolymer combinations are, for example, vinylidenefluoridelhexafluoropropene, optionally together with tetrafluoroethyleneand/or perfluoro(methyl vinyl ether), tetrafluoroethylene/ethylene,tetrafluoroethylene/vinylidene fluoride, tetrafluoroethylene/propene,tetrafluoroethylene/propene/vinylidene fluoride andtetrafluoroethylene/vinyl acetate.

It is additionally possible also to use copolymerizable monomerscontaining iodine or bromine, such as for examplebromotrifluoroethylene, 4bromo-3,3,4,4-tetrafluoro-1-butene, asdescribed in U.S. Pat. No. 4,035,565, or 1-bromo-2,2-difluoroethylenefor the production of peroxide-vulcanisable fluororubbers.

In a preferred embodiment of the invention, water is additionallypresent in a quantity of 10 to 900 parts by weight relative to 100 partsby weight of the polyfluoropropenes or polyfluorobutanes used accordingto the invention.

The free-radical polymerization is preferably initiated by means ofinitiators.

Organic or fluoro-organic dialkyl peroxides, diacyl peroxides, dialkylperoxy-dicarbonates, alkyl peresters and/or perketals, such as forexample tert.-butyl peroxypivalate, tert.-butyl peroxy-2-thylhexanoate,dicyclohexyl peroxydicarbonate, bis(trifluoroacetyl peroxide) or theperoxide of hexafluoropropene oxide dimers {(CF₃CF₂CF₂O CF(CF₃)COO}₂ arepreferably used as the initiator. The initiator which is used and thequantity in which it is used is determined by the particular reactiontemperature, at which the half-life of the peroxide to be selectedshould be between 30 and 500 min. Quantities of between 0.05 and 1.0part by weight of peroxide per 100 parts by weight of monomers to bereacted are preferred.

When the polymerization is performed as suspension or emulsionpolymerisation in the presence of water, which is additionally added tothe fluorinated hydrocarbons according to the invention, it is alsopossible to use water-soluble initiators or redox systems, in which oneor both of the components (reducing and oxidizing agent) arewater-soluble, such as for example potassium peroxydisulfate, potassiumpermanganate/oxalic acid or peroxydisulfate/perfluoroalkyl sulfinate.

The molecular weights and thus the viscosities of the desired productsmay be adjusted by means of the quantity of initiator or by addition ofone or more chain-transfer agents.

Preferably used chain-transfer agents are compounds of the formula (III)

R³Br_(a)I_(b)  (III)

where a or b=0 to 2 and a+b=1 or2, wherein R³ may be an aliphatichydrocarbon, fluorinated hydrocarbon, chloro-fluorocarbon orfluorocarbon residue having 1-8 carbon atoms. These may comprise, forexample, 1,2-dibromo-1-chlorotrifluoroethane and/or1-bromo-2-iodotetrafluoro-ethane. More preferred diiodo-organiccompounds are those in which both iodine atoms are attached to the sameor to different carbon atoms. Most preferred hydrocarbon or fluorocarboncompounds are those having 1 or 4 carbon atoms, wherein the iodine islocated on the terminal carbon atoms. Diiodomethane and/or1,4-diiodoperfluorobutane are likewise very preferred. The quantity ofdiiodo-organic compound is preferably 0.1-3.0 parts by weight of iodineper 100 parts by weight of polymerized fluoromonomer.

The polyfluoropropanes or polyfluorobutanes used in the processaccording to the invention preferably have an H/F ratio of 1/3 to 3/3.

Thanks to the particular distribution of the hydrogen atoms, it isensured that no transfers from the solvent occur which impair thereaction or product properties. If, for example,1,1,1,4,4,4-hexafluorobutane is used instead of the1,1,1,3,3-penta-fluorobutane used according to the invention,considerable retardation of polymerization is observed.

The polyfluoropropanes or polyfluorobutanes used according to theinvention are produced using known processes (c.f. for example Zh. Org.Khim. 1980, 1401-1408 and 1982, 946 and 1168; Zh. Org. Khim. 1988, 1558;J. Chem. Soc. Perk, 1, 1980, 2258; J. Chem. Soc. Perk Trans., 2, 1983,1713; J. Chem. Soc. C 1969, 1739; Chem. Soc. 1949, 2860; Zh. Anal. Khim.1981 36(6), 1125; J. Fluorine Chem. 1979, 325; Rosz. Chem. 1979 (48),1697, J. Amer. Chem. Soc. 67, 1195 (1945), 72, 3577 (1950) and 76, 2343(1954)). Since the boiling points of these fluorinated hydrocarbons arebetween approx. −1 and 40° C., they may readily be removed from theproduct on completion of polymerization. Any possible secondaryproducts, which may arise from the initiator and/or the chain-transferagents, have higher boiling points, such that the solvent may berecovered again in very pure form.

The reaction temperatures for the free-radical polymerization arepreferably between 30 and 130° C. Lower temperatures result in adramatic extension of running time and in a sharp increase in theviscosity of the polymer solution, such that problems may occur withregard to mass transfer, heat dissipation and product discharge. Thespace-time yield cannot be raised substantially firther with stillhigher temperatures, while product properties are degraded. A preferredtemperature range for polymerization is 60-100° C.

The pressure during polymerization is dependent upon the above-statedconditions and upon the composition of the monomer mixture and ispreferably between 10 and 100 bar. The process according to theinvention is more preferably performed at pressures of between 15 and 50bar.

The free-radical polymerization may be performed by batch, continuous orbatch/feed processes in stirred tank reactors, wherein the batch/feedprocess is preferred.

Once polymerization is complete, the reaction mixture may readily bedischarged or expressed from the tank via a bottom discharge or riserpipe. Residual monomers and the solvent may then readily be separatedfrom the polymer by releasing the pressure.

The polymer solution may, however, also be used without further workingup as a coating material, which use is also provided by the presentinvention. Substrates which may be considered for coating are, forexample, metals, plastics, textiles, leather, paper and nonwovens.

Due to their low boiling points, the solutions according to theinvention may also readily be sprayed onto substrate surfaces byintrinsic pressure. If curing is required after application, thecoatings according to the invention may be vulcanized using knownmethods, for example by free-radical methods by means of co-vulcanizingagents and light or peroxides, with polyamines or polyols and with theassistance of diisocyanates, if the polymer contains hydroxyl groups.

Fluororubbers produced using process according to the invention may becompounded and vulcanized using conventional methods, c.f. ModernFluoropolymers, John Wiley & Sons Ltd., Chichester, 1997, pp. 78, 115,601. Thanks to their low viscosity, the fluororubbers may be firtherprocessed to yield elastic moldings by using advantageous injectionmoulding techniques.

Compounds suitable for vulcanization are bisnucleophiles, such asbisamines, for example hexamethylenediamine, or bisphenols, for example2,2-bis(4-hydroxy-phenyl)hexafluoropropane (“Bisphenol AF”) incombination with vulcanisation accelerators, such as quaternaryphosphonium, ammonium or sulfonium salts and acid acceptors, such asmagnesium oxide and calcium hydroxide, c.f. A. L. Logothetis in Polym.Sci. 14 (1989) 251-296 and the literature cited therein. Alternatively,fluororubbers which have been produced by the process according to theinvention and contain bromine and/or iodine covalently bonded to thecarbon main or side chain, may be vulcanised by organic peroxides suchas 2,5-dimethyl-2,5-bis(tert.-butyl)hexane in combination withco-vulcanising agents such as triallyl isocyanurate (c.f. for exampleEP-A 398 241).

The present invention also provides the use of1,1,1,3,3-pentafluoropropane as process solvent for the polymerizationof fluoromonomers.

The following Examples illustrate the invention, but do not limit it.

PRACTICAL EXAMPLE Example 1

A sealed 4.1 L autoclave cooled to ≦5° C. was inertised by beingevacuated and purged three times with nitrogen. A solution of 836 g of1,1,1,3,3-pentafluoropropane and 18.6 g of1,2-dibromochlorotrifluoroethane was sucked in through a tube, likewiseprovided with an inert atmosphere. 440 g of vinylidene fluoride (VDF)and 1028 g of hexafluoropropene (HFP) were then added and the reactionmixture heated to 80° C. while being stirred. Once this temperature hadbeen reached, the internal pressure in the autoclave was 34 bar.Polymerization was initiated by adding 2.5 g of tert.-butylperoxy-2-ethylhexanoate (Peroxid-Chemie GmbH). Polymerization beganafter a few minutes, as indicated by the pressure beginning to fall.During polyrerization, a monomer mixture comprising 60 wt. % vinylidenefluoride and 40 wt. % hexafluoropropene was pumped in such that theinternal pressure in the autoclave was held constant at 34±0.4 bar. Inthis manner, a total of 303 g of vinylidene fluoride and 196 g ofhexafluoropropene were apportioned within a reaction time of 455 min.Once polymerization was complete, the unreacted monomer mixture wasremoved from the reactor by depressurization and evacuation. 15 minutesafter the stirrer had been turned off, the remaining contents of thereactor, assuming the form of a solution of the polymer in1,1,1,3,3-pentafluoropropane, were completely discharged via a bottomoutlet valve into a second pressure vessel located beneath. The polymersolution was dried for 24 hours at 60° C. in a vacuum drying cabinet,wherein the solvent was condensed in a cold trap, and 493 g of a highviscosity copolymer were obtained.

The following copolymer composition was determined by ¹⁹F-NMR analysis(solvent: acetone; standard: CFCl₃): 22.7 mol % hexafluoropropene, 77.2mol % vinylidene fluoride. The bromine content of the polymer,determined by elemental analysis, was 0.9 wt. %, while the chlorinecontent was 0.2 wt. %.

Molecular weights were determined by performing gel permeationchromatographic (GPC) measurements with RI detection indimethylacetamide (DMAC) at 40° C. with the addition of 1 g/l of LiBr.Evaluation was performed using a special calibration curve forpolyethylene oxide, which had been calibrated by membrane osmosismeasurements. The number and weight average molecular weights (Mn, Mw)are shown in Table 1.

Example 2

Polymerization was performed in a similar manner as in Example 1, butwith 12.0 g of diiodomethane as chain transfer agent instead of1,2-dibromochlorotrifluoroethane and with addition of 2.21 g oftert.-butyl peroxy-2-ethylhexanoate at the beginning of the reaction and1.1 g once a total of 300 g of monomers had been apportioned.

A total of 412 g of vinylidene fluoride and 264 g of hexafluoropropenewere apportioned within a reaction time of 1032 min. 714 g of a highviscosity copolymer could be isolated by working up in a similar manneras in Example 1.

The composition of the copolymer is 77.8 mol % hexafluoropropene, 22.2mol % vinylidene fluoride. The iodine content of polymer is 1.45 wt. %.

Table 1 shows the results of the GPC analysis.

A vulcanizable composition was produced by incorporating 30 parts ofcarbon black MT N 990, 3 parts of calcium hydroxide, 4 parts ofPerkalink 301/50 (triallyl isocyanurate, 50% on silica gel) and 3 partsof Luperco 101 XL45 (2,5-dimethyl-2,5-bis(tert.-butylperoxy)hexane; 45%in inactive fillers) into 100 parts by weight of the fluororubbercopolymer on a well cooled two roll mixing mill.

Vucanization behavior was determined by investigating the compositioncontaining peroxide in a Monsanto model MDR 2000 E rheometer at 170° C.(measurement time 30 min).

The composition was pressure vulcanized for 15 min at 170° C. and 200bar in molds to produce 1×10×10 mm sheets and then post-vulcanised in acirculating air oven (1 h at 160° C., 1 h at 170° C., 2 h at 180° C. and20 h at 230° C.). Tension/elongation properties were determined on thevulcanized mouldings. The results are listed in Table 2.

Example 3

Polymerization was performed in a similar manner as in Example 1, butwithout addition of a chain-transfer agent.

A total of 182 g of vinylidene fluoride and 116 g of hexafluoropropenewere apportioned within a reaction time of 660 min. 292 g of a rubberycopolymer could be isolated by working up in a similar manner as inExample 1. Copolymer composition was 77.8 mol % VDF, 22.2 mol % HFP.Table 1 shows the results of the GPC analysis.

A vulcanizable composition was produced by incorporating 30 parts ofcarbon black MT N 990, 6 parts of calcium hydroxide, 3 parts ofmagnesium oxide (Maglite D) and 4 parts of a mixture of bisphenol AF andViton A (50:50 parts by weight) and 2 parts of a mixture ofbenzyltriphenylphosphonium chloride with Viton A (33:66 parts by weight)into 100 parts by weight of the fluororubber copolymer on a well cooledtwo roll mixing mill.

The results of vulcanization testing are also shown in Table 2.

Example 4

1155 g of 1,1,1,3,3-pentafluoropropane, 0.3 g of tert.-butylperoxy-2-ethylhexanoate and 18 g of diiodomethane together with amonomer mixture of 416 g of vinylidene fluoride, 1170 g ofhexafluoropropene and 71 g of tetrafluoroethylene were initiallyintroduced in a similar manner as in Example 1 with the 4.1 L autoclavebeing cooled to ≦5° C. Once the reaction mixture had been heated to 80°C., the internal pressure in the autoclave was 34 bar. After a reactiontime of 3 h at 80° C., 19.9 ml of a solution of tert.-butylperoxy-2-ethylhexanoate in 1,1,1,3,3-pentafluoropropane (50 g/1) wereapportioned. In order to maintain the initial pressure, a monomermixture consisting of 53 wt. % of vinylidene fluoride, 34 wt. % ofhexafluoropropene and 13 wt. % of tetrafluoroethylene was apportioned.Once a total of 300 g and 600 g of monomer had been apportioned, afurther 12.4 ml of the above-stated peroxide solution was added in eachinstance.

A total of 599 g of vinylidene fluoride, 283 g of hexafluoropropane and108 g of tetrafluoroethylene was consumed within a reaction time of 859min. 1025 g of a highly viscous copolymer could be isolated by workingup in a similar manner as in Example 1.

Copolymer composition was 70.8 mol % of VDF, 18.4 mol % of HFP and 10.8mol % of TFE. Iodine content was 1.6 wt. %.

Example 5

In a similar manner as in the preceding Examples, 836 g of1,1,1,3,3-pentafluoro-propane, 660 g of vinylidene fluoride wereinitially introduced into the 4.1 L autoclaves. Polymerization wasinitiated at 80° C. and a pressure of 41 bar by addition of 2.5 g oftert.-butyl peroxy-2-ethylhexanoate. 394 g of vinylidene fluoride wereapportioned within 20 h while maintaining the internal pressure in theautoclave.

408 g of a VDF homopolymer powder were isolated having a DSC meltingpoint of 167° C.

Comparative Example 1

Bulk Polymerization

15.3 g of diiodoperfluorobutane (DIPFB, Fluorochem Ltd., after priorpurification by extraction with aqueous sodium thiosulfate solution),771 g of VDF and 2118 g of HFP were initially introduced into a 4.1 Lautoclave, which had been rendered inert by repeated evacuation andnitrogen depressurisation, and the mixture heated to 60° C. while beingstirred. Once this temperature had been reached, the internal pressurein the autoclave was 37.7 bar. Polymerization was initiated by adding4.25 g of tert.-butyl peroxypivalate in the form of TBPPI-75-AL(solution in aliphatic compounds, peroxide content 47.1%, Peroxid-ChemieGmbH).

A total of 308 g of vinylidene fluoride and 202 g of hexafluoropropenewere apportioned within a reaction time of 1779 min. Once polymerisationwas complete, the unreacted monomer mixture was removed from the reactorby depressurisation and evacuation. The polymer could not be dischargedfrom the autoclave by simple draining as in Examples 1-3. The polymerwas removed by being dissolved in 3 L of acetone and precipitated fromthis solution with n-hexane. After drying, 481 g of a highly viscouscopolymer were obtained with the composition 76.7 mol % VDF and 23.3 mol% HFP and 1 wt. % iodine.

A vulcanizable composition was produced in a similar manner as inExample 1. The results are also shown in Table 2.

This Comparative Example shows that, despite initially introducing asubstantially larger quantity of monomer, when polymerization isperformed in pure liquid monomer phase, a distinctly lower space-timeyield is achieved and discharging the product is highly problematic.Moreover, chain transfer is clearly rendered more difficult, resultingin lower incorporation of iodine and poorer vulcanisability.

Comparative Example 2

(Use of an alternative fluorinated hydrocarbon:1,1,1,4,4,4-hexafluorobutane as polymerisation medium, similar to U.S.Pat. No. 5,182,342)

Polymerisation was performed in a similar manner as in Example 2, butusing 829 g of 1,1,1,4,4,4-hexafluorobutane instead of the1,1,1,3,3-pentafluoropropane. The internal pressure in the autoclave was29 bar once the initial quantity of monomers had been introduced and thereaction temperature of 80° C. had been reached.

A total of 214 g of vinylidene fluoride and 147 g of hexafluoropropenewere apportioned within a reaction time of 1320 min.

As in Examples 1 and 2, once the residual monomers had been removed, thereaction mixture assumed the form of a uniform polymer solution, fromwhich the solvent was removed by distillation. 387 g of a copolymercomprising 78.1 mol % VDF and 21.9 mol % HFP and having an iodinecontent of 2.2 wt. % are obtained.

A vulcanizable composition was produced in a similar manner as inExample 1. The results are also shown in Table 2.

This Comparative Example shows that the space-time yield is distinctlylower when a fluorinated hydrocarbon is used which differs from thataccording to the invention with regard to the distribution of thehydrogen atoms.

Comparative Example 3

Polymerization similar to Example 2, using trifluoroethanol (used inExamples 7, 25, of U.S. Pat. No. 5,182,342)

In a similar manner as in Example 1, 438 g of 2,2,2-trifluoroethanol and7.6 g of diiodoperfluorobutane together with a monomer mixture of 289 gof vinylidene fluoride and 666 g of hexafluoropropene were initiallyintroduced into the 4.1 L autoclave. Once the reaction mixture had beenheated to 80° C., the internal pressure in the autoclave was 35 bar.Polymerization was initiated by adding 1.24 g of tert.-butylperoxy-2-ethylhexanoate. After an initially brisk reaction, the reactioncame to a standstill after approx. 10 h. Up to that point, 102 g ofvinylidene fluoride and 44 g of hexafluoropropene had been apportionedto maintain the internal pressure in the autoclave.

The test was terminated after 20 h, the unreacted monomer mixturedepressurized and the reaction mixture discharged. The resultant polymerwas not dissolved in the trifluoroethanol, but instead assumed the formof a swollen, separate phase. The yield was 185 g of a highly viscouscopolymer.

This Comparative Example shows that trifluoroethanol is clearly notsufficiently inert during the polymerisation and secondary reactionsoccur which result in termination of the reaction.

TABLE 1 GPC results Example 1 2 3 Mn 15.5 17.4 39.6 Mw 28.5 27.1 68.4

TABLE 2 Vulcanization results and properties of the vulcanizatesComparative Example Example 2 3 4 1 2 MDR results s′ min [dNm] 0.02 0.060.01 0.03 0.02 s′ max (dNm) 12.8 13.5 21.5 12.4 11.1 tan δ_(max) 0.090.15 0.08 0.16 0.07 t 90 [min] 6.4 2.2 6.5 4.6 9.2 Mechanical Tensilestrength [N/mm²] 10.9 10.0 12.6 n.d. 7.5 Elongation [%] 172 256 164 n.d.96 S₅₀[N/mm²] 2.7 2.5 3.3 n.d. 3.4 Compression set 37 33 32 n.d. n.d.(70 h/200° C.) [%] n.d.: not determined

What is claimed is:
 1. A process for the production of partiallyfluorinated fluoropolymers, wherein fluoromonomers are free-radicallypolymerized in the presence of fluorinated hydrocarbons of the formulaeCF₃—CH₂—CF₂H, CF₃—CHF—CF₂H, CF₃—CH₂—CF₃, CHF₂—CF₂—CH₂F, CF₃—CH₂—CF₂—CH₃and/or CF₃—CHF—CF₂—CH₃, and optionally a chain-transfer agent.
 2. Aprocess according to claim 1, wherein water is additionally presend. 3.A process according to claim 1, wherein at least one initiator in theform of organic or fluoroorganic dialkyl peroxides, diacyl peroxides,dialkyl peroxydicarbonates, alkyl peresters and/or perketals isadditionally used.
 4. A process according to claim 1, wherein said chaintransfer agent is at least one compound of the formula (III)R³Br_(a)I_(b)  (III) where a or b=0 to 2 and a+b=1 or 2, wherein R³ maybe an aliphatic hydrocarbon, fluorinated hydrocarbon, chlorofluorocarbonor fluorocarbon residue having 1-8 carbon atoms.
 5. A process accordingto claim 1, wherein said fluorinated hydrocarbon is1,1,1,3,3-pentafluoropropane.